OK, let’s drop back 50 yards and punt like HE double L.
(1) Looking @ manual your memory should be in the 2 and 4 the slot (counting from one nearest the CPU (This would be A1 and B1 slots)
(2) What is the status of your mem ok led – should be off.
(3) What is your current Bios version (can find on CPU-Z Mainboard tap. When having a problem it is always a good idea to have the latest Bios version.
...A) There are 3 versions of the P7P55D-E. Deluxe Latest bios is 1504, the Pro Latest Bios 1602 and the Premium, didn’t look up. MAKE sure you get the right one.
..B) To update bios if you do not have latest – follow Para 3.2.2 in manual (Asus EZ Flash 2 Utility.
..C) BEFORE updating Bios, as you are having a memory problem, I would recommend that you download Memtest 86.iso and make a bootable ISO disk with memtest on it – boot to disk and run for at least 2 hours. If you have any mem errors, remove memory stick from B1 and just use the one in A1. Retest, should be OK.
(4) After updating the Bios see if this fixed your problem (boot to win 7 and check memory status)
(5) Still have a problem.
..A) Is mem LED on, if so try using Mem ok (sec 2.6 in manual.
..B) Try loading XMP; this is located in bios under the AI Overclocker Tuner (Section 3.5.2 of manual.
Added: link to metest86 ISO
http://www.memtest86.com/download.html